Automatic gain control or AGC systems are known and widely used. The purpose of the AGC system is to adjust the level of a received signal in the receiver at an input of an analog-to-digital converter (ADC). The AGC system controls the receiver gain in order to keep the signal level at the ADC input as close as possible to a reference level, in order to avoid saturation of the ADC, whilst ensuring that the full range of the ADC is utilized to thereby provide the required signal to noise ratio (SNR) in the sampled signal. A further purpose of the AGC is to maintain the level of the receiver baseband (BB) output signal as close as possible to a reference level.
The general principle of AGC systems comprises measuring the received signal and using that measurement in computing and setting a required gain for the receiver. Generally, an average power may be measured over a certain duration of time and the measured average power then used in the AGC system. The duration of time over which the average power measurement is taken determines the rate at which the AGC system updates the receiver gain.
An example radio frame structure, such as may be used in a network connection in an EUTRAN communication system, is illustrated in FIG. 1. In the example radio structure, each radio frame is split into 10 sub-frames, with each sub-frame comprising two slots. The transmitted signal in each sub-frame may comprise a number of resource blocks (NRB) and a number of OFDM symbols in each resource block (FIG. 2). Each transmitted resource block (RB) consist 12 sub-carriers and 14 OFDM symbols with a prefix. The first and fifth OFDM symbols of each slot comprise two permanent common pilot (PCP) sub-carriers, with the remaining sub-carriers available for use as data sub-carriers. These symbols comprising PCP sub-carriers are called pilot symbols. These act as reference symbols and are known symbols. The third, fourth, sixth and seventh OFDM symbols may contain data sub-carriers. These symbols are named as the data symbols. However, further pilot symbols may be present in the received signal due to signals transmitted from other transmitting antennas in the cell.
The minimum time required for correct measurement of the OFDM symbol power is determined by the sub-carrier spacing. In EUTRAN wireless telecommunication systems the sub-carrier spacing is 15 kHz and the minimum time required for correct measurement of the OFDM symbol power is 66.66 μs. However, a 66.66 μs duration time may be too short to be suitable for the updating time in a EUTRAN system.
For a EUTRAN system the average power measurement duration time, and correspondingly the gain updating time, may be chosen equal one sub-frame (for example 1 ms) as in a WCDMA receiver. Therefore, for a EUTRAN system the AGC system may measure the average power for the current sub-frame and use this measured value to calculate and set the required gain for the next sub-frame.
The transmitted average power of the signal measured over a sub-frame duration may vary from sub-frame to sub-frame. The average power variation may depend on the employed number of resource blocks (NRB). The minimum number of resource blocks may be zero. The maximum value of NRB depends on the transmission bandwidth configuration in the cell and may vary, for example from 6 to 100. The transmission bandwidth configuration, i.e. the maximum value of NRB may be known. However, the number of resource blocks in use during any one sub-frame depends on wireless call scheduling and may not be accurately predicted in advance.
FIG. 3 shows one example of the transmitted EUTRAN signal which consists of two sub-frames. The first sub-frame comprises only the permanent common pilot signals, and no data transmissions. The average power of the signal in the first sub-frame is given by:
                              P                      av            ⁢                                                  ⁢            1                          =                              2            ·                          P              PCP                        ·                          N              RB                        ·            4                    14                                    i        .            where PPCP is the power received due to a single sub-carrier.
For the second sub-frame all available data sub-carriers are in use. The average power of the signal in the second sub-frame is given by:
                              P                      av            ⁢                                                  ⁢            2                          =                              12            ·                          P              PCP                        ·                          N              RB                        ·            14                    14                                    i        .            
The ratio of the average powers of the first and second sub-frames is therefore:
                              10          ·                      log            ⁡                          [                                                P                                      av                    ⁢                                                                                  ⁢                    2                                                                    P                                      av                    ⁢                                                                                  ⁢                    1                                                              ]                                      =                  13.2          ⁢                                          ⁢          dB                                    i        .            
Thus, the range of the average power variations in this case is 13.2 dB. These average power variations can result in the receiver gain fast switching, which is undesirable. Therefore, problems exist in using known AGC systems for EUTRAN receivers when the average power measured in the time domain is used for the gain calculation and setting.
Furthermore, adjusting the gain of the EUTRAN receiver based on the average power of the received signal requires extra headroom to be allowed for the analog-to-digital converter, in addition to the peak-to-average-ratio (PAR) and fading headroom, since the power of the OFDM symbols may exceed the sub-frame average power. For example for the first sub-frame (FIG. 3) the average power is Pav1 and the power of the PCP symbols is equal 2×PPCP×NRB. The ratio of the PCP symbol power to the sub-frame average power Pav1 is 5.4 dB. Therefore, the ADC should be allowed an extra headroom of 5.4 dB in order to avoid the clipping of the signal. This extra required headroom may result in a decrease in the required reference signal level at the ADC input, and therefore in a reduced signal-to-noise ratio (SNR) at the receiver output.
The receiver may also receive the desired signal associated with a local cell along with a strong interfering signal at an adjacent channel from a neighbouring cell. The interfering signal may be a GSM, WCDMA or EUTRAN signal. The average power of this interfering signal, or EUTRAN blocker, at the adjacent channel may vary in a similar way as the average power of the desired EUTRAN signal. Therefore, taking into account the contribution of this blocker signal to the received signal power, the power variation at the receiver input may increase in the presence of a blocker signal.
A known AGC system is described in U.S. 2006/0222118, “Automatic Gain Control for wireless receiver”. In the described AGC system, an average power is measured with the aid of a power detector in the time domain, and this average power is used to control the adjustment of the LNA gain and the digital variable gain amplifier (DVGA).
A further known AGC system is disclosed in U.S. 2006/0222116, “AGC with integrated wideband interferer detector”. In this AGC system, the average received power is measured with the aid of the RSSI calculator and the symbol power measured with the aid of the integrated wideband interferer detector. Both of these power measurement blocks measure the power in time domain. The average received power is used to control the adjustment of the LNA gain and the post mixer amplifier (PMA) gain.
A further known AGC system is disclosed in U.S. 2005/0250462, “Gain control circuit”. In the described AGC system, the average received power is measured with the aid of a power detector in the time domain, and is used to control adjustment of the LNA gain and the base-band amplifier (BBA) gain.
All of these known AGC systems use the average power measured in the time domain to control adjustment of the gain and do not predict the variations in power of the required channel in a EUTRAN system when the number of resource blocks in use, NRB, in not known. This problem may result in the EUTRAN receiver gain fast switching if any of these AGC systems were to be used in EUTRAN receiver.
It is an aim of some embodiments of the present invention to address, or at least mitigate, some of these problems.